Beam forming for smart antennas may reduce multi-access interference in a CDMA (Code Division Multiple Access) system and improve system capacity. When an interference signal is measurable, a spatial covariance matrix of the interference signal may be obtained. Suppressing interference signal is considered during beam forming weight coefficient calculation, so that an optimal interference suppressing effect can be achieved.
When a Time Division-Code Division Multiple Access (TD-CDMA) system is receiving signals, received expected signals and interference signals are all in the same slot. The received uplink expected user signals and the received uplink interference user signals may be directly used to calculate the weight coefficient of the receiving beam forming. When a TD-CDMA system is transmitting downlink beam forming, the received uplink expected user signals and the received uplink interference user signals are commonly used to estimate the weight coefficient of downlink transmitting beam forming. Taking the method for downlink beam forming shown in FIG. 1 as an example, in Step S101, spatial covariance matrixes of expected user signals and spatial covariance matrixes of interference user signals can be estimated based on some received uplink expected user signals and interference user signals in the slot in which the expected user signals locate; in step S102, based on the spatial covariance matrixes of the expected user signals and the spatial covariance matrixes of the interference user signals, the expected users' beam forming weight coefficients may be calculated according to certain rules (for example, maximal signal to noise ratio rule or maximal signal to interference ratio rule, etc.); in Step S103, transmitting beam forming can be implemented based on the obtained expected users' weight coefficients.
However, the above mentioned method is based on an assumption that the uplink/downlink expected users and the interference users are completely symmetrical, namely, the expected users and the interference users in some uplink time-slot are exactly the same with the expected users and the interference users in the corresponding downlink slot. The time slot structure and the allocation method of the uplink/downlink time slots are shown in FIG. 2, wherein the interference users may be considered as the users from other cells. The uplink expected users 1, 2 and the uplink interference users 1′, 2′, 3′ are in time slot 1, and the downlink expected users 1, 2 and the downlink interference users 1′, 2′, 3′ are in time slot 4, namely, the uplink time slot 1 and the downlink time slot 4 are corresponding to each other.
In respect to a TD-CDMA system having a symmetry relationship, transmitting beam forming certainly can be implemented according to a traditional method. However, services in a TD-CDMA system are sometimes asymmetric; the amount of uplink/downlink time slots should be flexibly configured and can not be fixed according predefined allocation method. Furthermore, flexible channel allocation scheme may allocate users' uplink/downlink time slots without a fixed corresponding relationship. In these cases, the above mentioned traditional beam forming method aiming to suppressing interference is not feasible any more.